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Depends - if it's just a fun gun I'd probably shoot it and see what happens. At a worst case your zero will wander as the barrel (and front sight) wiggles around, or the piston will bind and the rifle won't cycle. You can always rebarrel with a shim later.

If you do, make sure to shoot some groups with the 40ft-lb torque, and then again with the 120 ft-lb torque. There's been a lot of discussion and speculation about the affect of barrel torque on accuracy, and I'd be curious to see your results.

I referenced this thread the other night while replacing the barrel on my Imbel. It hand timed (no tools) to about 11:30. And it timed to 12:00 with about 30 ft-lbs. I was not comfortable with this so I measured the my two barrels. The best I could figure was a difference between the shoulder and breach face between the two of about .001-.003"
I found a "breaching washer" / shim laying around my dad's wood shop that had a thickness of about .001" so I cut a hole out of it that would fit around the threads and then it hand timed to about 10:45. Used a torque wrench to time it to 12:00 and it's right around 110 ft-lbs now. Actually it's about 12:10 so I need to back it off a bit. But the shim worked fine. It was made by a company called "Natural Light" in case anyone was curious.

I referenced this thread the other night while replacing the barrel on my Imbel. It hand timed (no tools) to about 11:30. And it timed to 12:00 with about 30 ft-lbs. I was not comfortable with this so I measured the my two barrels. The best I could figure was a difference between the shoulder and breach face between the two of about .001-.003"
I found a "breaching washer" / shim laying around my dad's wood shop that had a thickness of about .001" so I cut a hole out of it that would fit around the threads and then it hand timed to about 10:45. Used a torque wrench to time it to 12:00 and it's right around 110 ft-lbs now. Actually it's about 12:10 so I need to back it off a bit. But the shim worked fine. It was made by a company called "Natural Light" in case anyone was curious.

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Why would the firing increase load? The firing is trying to unscrew the barrel, is it not? It seems to me the initial load is to overcome the desire to unload.

I think the friction of the bullet moving down the barrel would increase friction on the threads as the barrel tries to pull away from the receiver, increasing load on the threads of the barrel, but at the same time the rotation of the bullet would be trying to unscrew the barrel. I'm not sure which force would overcome the other, nor what the minimum torque would be to overcome the rotational force, but I figured input is input. I stick with the "good-and-tight" method myself.

Given that the gas cylinder support nests into the crescent notch on the barrel flange, is it not, in fact, impossible for the barrel to ever unscrew ?

I understand that the preload on the barrel threads is reduced at 40ftlb compared to 120ftlb, so that joint might be a little less stiff at 40ftlb than at 120ftlb, but one is surely at a zero backlash condition already at 40ftlb ?

In looking at torque specifications for other fasteners of a similar size yesterday, it seems that around 60Nm is a common torque spec, which is close to 45ftlb but in most cases they use a lubricant whereas the barrel is assembled dry. For fasteners that have to be "tight" it is far more common that a specific angular rotation be specified, which translates to a defined "stretch" in the fastener. The Fal barrel torquing procedure in fact follows this definition, but the threaded portion if the barrel is very short which is why there is so much discrepancy in the preload value (compare the length of a cylinder head bolt to the length of thread engagement of the barrel). Varmint Al's web page demonstrates nicely how non uniform the stress is on a threaded joint.

A fastener the same size as a fal barrel threaded breech end has a torque spec of 45 ftlbs? Haha thatd be cute

And btw shot it today and its the worst accuracy of any ive shot, put my .060 shim on and im at 11:50 and 100ftlbs.......gotta go get/borrow a better vise at 5. This barrel was from a member who had better accuracy out of it than ive achieved so there probably is something to the claim of under torqued barrels and accuracy concerns.

45 ft*lbs on a 1"x16 TPI thread? Ok, but jeez, what are the fasteners made of, cheddar cheese?

Mark, the firing will increase load on the threads as chamber pressure divided by the area roughly equivalent to the area of the cartridge shoulder will increase the load on threads.

This is tough to calculate is there is also the friction between the chamber walls and the brass case, but if friction is ignored then the initial axial force would be equal to the pressure times the cartridge head area, not the shoulder area. As the bullet travels down the bore this pressure and force decrease, and the rotational forces come into play. The axial forces are high enough that I would imagine the rotational forces would be negligible in comparison.

Kind of an "angels on the head of a pin" argument, but I couldn't help myself. ;-)

My head hurts (shut up shlomo). I thought the FAL went from RH threads at the muzzle to LH threads because firing tended to loosen the RH flash hider. What is different about the forces applied to the barrel that would not make it also true for that?

I am not saying the barrel would unscrew, only that force 9stretching of threads) would apply to the barrel as well, making firing relieve rather than increase the stress.

Dangit - curiosity got the better of me and I had to run the numbers. Ignoring friction and simplifying this as much as possible, for a 1:12 twist barrel at 62,000 psi the bullet would provide about 11 ft-lbs worth of torque on the barrel. In theory as long as you torqued the barrel/reciever to more than 11 ft-lbs it will not unscrew.

Of course, friction plays a huge part here and cannot be ignored, but bringing friction into the equation will only make that number smaller. Either way - I'm pretty confident that you're not going to unscrew your barrel by shooting it.

ok, since you apparently are one of the "smart guys" (meant as a compliment) maybe you can figure out the amount of torque in ft pounds that a 9/16x24 muzzle device will create if you hand time to 15 minimum and 30 degrees maximum short of BDC. Or do you need the barrel steel modulus of elasticity as a constant?

I'm going to print this out and frame it, and pull it out the next time I feel the cudgel. Even better - I'll hang it on shlomo's wall just to gloat.

I'm afraid there are just too many variables to answer your question though. You'd really be better off taking a few examples and using a lever-arm torque wrench to see how much torque it takes to turn it that final 15° or 30°.

Torque angle is a much more accurate way of approximating clamping force or bolt stretch (what you are really trying to measure when tightening threaded fasteners, but usually unable to do so directly) than torque wrench readings. I could make a bunch of assumptions and throw out a number, but even if I got all the assumptions right (a REALLY big if), the figures could still be off as much as 30%. It's kind of like asking me to convert 0.0156" that you'd read with a micrometer to 1/64" so you can try and read it with a pastic ruler.

There's at least a 30% error built into my 11 ft-lb number above as well, but it should be around the right order of magnitude.

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Last edited by kotengu; January 07, 2013 at 18:57.
Reason: To correct my grammar before shlomo bursts a blood vessel.

My head hurts (shut up shlomo). I thought the FAL went from RH threads at the muzzle to LH threads because firing tended to loosen the RH flash hider. What is different about the forces applied to the barrel that would not make it also true for that?

I am not saying the barrel would unscrew, only that force 9stretching of threads) would apply to the barrel as well, making firing relieve rather than increase the stress.

A RH twist wants to tighten both the R/H barrel/receiver joint, and the L/H F/H joint. Visualize both ends from the rear, noting which way they turn relative to the barrel to unscrew. Now turn the barrel counterclockwise (viewed from the rear) Both ends tighten.

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Back to your barrel - how many shots did you fire with the barrel torqued to 40 ft-lbs, and did it easily screw and unscrew by hand still? Does the "hand tight" still come to the same place? Were you shooting iron sights or scoped?

6-8 MOA FALs are not unheard of, but definitely not to spec. I'm curious to see what your groups do with a properly torqued barrel.

At 40l ftbs no it doesnt unscrew by hand easily, but i have a feeling that a screw driver through the front sight ears and a hard sharp tug would do it haha but yes hand tight still in the same spot.....relatively as there are no marks and only memory to go by.

No - I meant when you unscrewed it (after you broke it loose), installed the shim, and re-installed it did it thread on easily and smoothly, or were the threads slightly deformed and it was harder to hand-screw back into place?

It shouldn't have been any different - I'm just double-checking my theory with a little reality.

I don't mean to rag on you too much kotex, you damn techie, but you got me.

Yeah, I probably should have included the bullet area, so same as case head area basically.

Mark, the torque to break the barrel loose at the breech end is larger (due to thread size/area and torque) than that of the muzzle break. Doesn't really matter on hand of threads. I don't know about the hand of the threads and loosening at the muzzle. It makes some sense in that the RH threaded FH's mostly have locking features.... except for maybe the long Browning flash hider?

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http://www.varmintal.com/amode.htm
"This is a .308 caliber 1.25" to 0.9" diameter straight taper stainless steel barrel 22" long with the action end fixed. You are looking at the muzzle end and it is tilted down slightly for better viewing. These are the first 8 mode shapes and frequencies. Each bending mode (like Mode #1) is on one plane, but there was another identical mode in another plane at the same frequency that was not shown to save space. The torsional modes #4 and #8 are at a high frequency. I should have printed the mesh lines on the torsion modes so one could see the rotation. Note, modes 1, 2, 3, 6, and 7 are shown in a single plane, but can exist in other planes. Modes below a frequency of about 500 Hz will not be able to complete one full cycle before the bullet exits the barrel."
Mode 1 82hz
Mode 2 406hz
Mode 3 1050hz
Mode 4 1756hz
Mode 5 1984hz
Mode 6 2485hz
Mode 7 3180hz
Mode 8 4171hz

No - I meant when you unscrewed it (after you broke it loose), installed the shim, and re-installed it did it thread on easily and smoothly, or were the threads slightly deformed and it was harder to hand-screw back into place?

It shouldn't have been any different - I'm just double-checking my theory with a little reality.

It did, no noticable change in friction. Still very easy to thread by hand after torquing to 40ftlbs........as i would have expected. Now at my current torque i still would t expect if i removed it for it to have deformed the threads.

I feel all threads have some elasticity and can return to learned heat treated position under normal conditions.

http://www.varmintal.com/amode.htm
"This is a .308 caliber 1.25" to 0.9" diameter straight taper stainless steel barrel 22" long with the action end fixed. You are looking at the muzzle end and it is tilted down slightly for better viewing. These are the first 8 mode shapes and frequencies. Each bending mode (like Mode #1) is on one plane, but there was another identical mode in another plane at the same frequency that was not shown to save space. The torsional modes #4 and #8 are at a high frequency. I should have printed the mesh lines on the torsion modes so one could see the rotation. Note, modes 1, 2, 3, 6, and 7 are shown in a single plane, but can exist in other planes. Modes below a frequency of about 500 Hz will not be able to complete one full cycle before the bullet exits the barrel."
Mode 1 82hz
Mode 2 406hz
Mode 3 1050hz
Mode 4 1756hz
Mode 5 1984hz
Mode 6 2485hz
Mode 7 3180hz
Mode 8 4171hz

Close up of mode 8 (torsion mode)

Things are never as simple as they seem...

Fascinating, thanks for sharing that

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It did, no noticable change in friction. Still very easy to thread by hand after torquing to 40ftlbs........as i would have expected. Now at my current torque i still would t expect if i removed it for it to have deformed the threads.

I feel all threads have some elasticity and can return to learned heat treated position under normal conditions.

Yep - I was more worried about the slight chance firing it without proper pre-load could have deformed the threads. If only a single thread was fully engaged it COULD have deformed when fired until it met up with a few other threads. I didn't really think this would happen, but it's always nice to know for sure. Thanks for the updates -

Yep - I was more worried about the slight chance firing it without proper pre-load could have deformed the threads. If only a single thread was fully engaged it COULD have deformed when fired until it met up with a few other threads. I didn't really think this would happen, but it's always nice to know for sure. Thanks for the updates -

When I chase threads, I crank the barrel down to around 100 ft pounds and break it free - 3 times. This is to seat any thread that might be a bit off. It bends the threads into fuller contact. Then I look at the timing.

I once did an Israeli where I chased the M25x1.5 to 1"x16. I did not reseat the threads as above. After 20 rounds, the barrel was loose.

1. What GP is talking about is the thread deformation from tightening, which is a good thing as long as you don't tighten too far and break anything. The threads slightly deform until they more evenly make contact and spread the load. What he's doing is smart in general. I'm curious though (Mark) - are you uniforming (cutting) the threads after doing this, or just using this process to uniform and make sure you're timing right?

2. There's around 10,000 lbs of force on the bolt face upon firing, that the receiver/barrel threads need to hold. As long as the clamping force (also called pre-load sometimes) of that threaded connection is greater than 10,000 lbs everything will stay tight as it's designed to. If not, things start coming apart to varying degrees. Your easy threading and rethreading shows that this didn't happen enough to matter, which is a good thing.

3. (sort of related) Thread relaxation occurs in any threaded joint. If you torque a barrel to 100 ft-lbs like GP did, then leave it over night - it will most likely take less than 100 ft-lbs to break it loose. That's because the threads (and possibly some other things) have deformed a little from that clamping force and the pre-load is not the same as when it started. This effect is magnified with big temperature swings, which happens every time you fire a shot. The "shock" of firing really doesn't have much to do with it, but abrupt forces do have a way of jiggling things loose that were on the edge before.

Probably more than you wanted, but it's all part of the reason for the higher torque specs on a barrel. It's not that it "won't work" at the lower specs, but the higher specs give a bigger margin for error.

1. What GP is talking about is the thread deformation from tightening, which is a good thing as long as you don't tighten too far and break anything. The threads slightly deform until they more evenly make contact and spread the load. What he's doing is smart in general. I'm curious though (Mark) - are you uniforming (cutting) the threads after doing this, or just using this process to uniform and make sure you're timing right?

You know, but others might not. There are thread designations - pitch, and angle. Most are 60degree, but there is also ACME (flat) and Witworth (55degrees like on mausers). Then there is classifications of thread, referring to the tolerance of the D(maj) and D(min) Or the diameter top of thread to opposing top of thead, and diameter bottom of thread to bottom of thread. "Class A" refers to male, and "class B" to female. 1A is a loose fit, such as an AK flash hider, designed to be easily removed by hand, in the field. 2A is normal and 3A a tight tolerance. The FAL is supposed to be 1"x16tpi, 60 degree, Class 2A.

For whatever reason, I sometimes end up with receiver to barrel combinations that are "too tight". I should be able to turn the barrel by hand - presuming it is free from caked oil and dirt. Sometimes I can't. My 1"x16 bottom tap is a set size, I can't change it. But my 1"x16 die is slightly adjustable. So I first chase the receiver threads with a tap. If it is still tight, I chase the barrel threads with the die at its largest opening. Usually, the cause is a flattened thread somewhere - causing the 60 degree angle of the thread to be mushed down a bit, and although shorter than the average D(maj) is is "wider". Sometimes that isn't enough either.

So I tighten the die a bit more and chase the threads again. I'll often see one side (leading or trailing edge) getting shaved, but not the top or other side. So maybe the lead screw in threading was off by a hair? Who knows. But I'm not going to muscle it on. For whatever reason, it isn't 16 threads per inch but maybe 16.01 tpi. Or maybe the cutter was a little off, making the thread 59 or 61 degrees?

After chasing them, they screw together pretty well by hand. But if the (leading or trailing) edge of the barrel thread is now different than the receiver, I may not have every thread engaging. Does it matter? Probably not. I've heard that 3 complete threads at 60% engagement is more shear strength than the shank that is threaded. Anyway, by cranking it down a couple time, I "form" the male and female threads to each other just a little more than they were before. While it may make no practical difference, it satisfies my immense ego to know it is as good as it can be and I can stamp my mark on it.

I suspect there is some minute deformation of the threads during firing. When unbreeching a typical IMBEL barrel from the receiver stub, I need a 4' breaker bar and a 20 ton press. I hop up and stiff-arm the breaker bar, adding my svelte and debonair 225 pounds to the bar. Often I hear a "pop" and then can unscrew by hand. Other times I have to use the 14" 1/2 drive for many rotations. Since this is obviously more than the 120 ft pound target, one of two things has happened.

1. IMBEL uses a hydraulic ram and cranks that barrel on regardless of how much pressure it takes or

2. The stretching of the threads during operation deforms them enough that they no longer match the original tolerance and classification and I have to force the deformed part through the non-deformed part. Given the differential heat treat of the barrel and receiver, I assume the deformation is on the barrel side. or

Good explanation - thanks. For some reason when you said "chase threads" I was thinking you were truing them as you would a custom Remington 700. I don't think I've ever heard of anyone going to that much trouble with a FAL, except for shlomo's custom barrel he played with a bit. I was trying to get straight in my head what would happen if you torqued, then re-cut a uniform thread. Of course cleaning up existing threads you'd be pretty limited in what you could do, but you got me thinking. My mistake - I'm going down the wrong road as usual!

Quote:

Originally Posted by gunplumber

I suspect there is some minute deformation of the threads during firing. When unbreeching a typical IMBEL barrel from the receiver stub, I need a 4' breaker bar and a 20 ton press. I hop up and stiff-arm the breaker bar, adding my svelte and debonair 225 pounds to the bar. Often I hear a "pop" and then can unscrew by hand. Other times I have to use the 14" 1/2 drive for many rotations. Since this is obviously more than the 120 ft pound target, one of two things has happened.

1. IMBEL uses a hydraulic ram and cranks that barrel on regardless of how much pressure it takes or

2. The stretching of the threads during operation deforms them enough that they no longer match the original tolerance and classification and I have to force the deformed part through the non-deformed part. Given the differential heat treat of the barrel and receiver, I assume the deformation is on the barrel side.

Number one seems much more reasonable than number two, but I hate to bad-mouth a factory and it's QC process without ever being there. Corrosion could also play a big part of course on some of our harder used rifles.

Great link, and the link behind that link (to the rest of the study showing the muzzle tuner details) is even better. Add all the screwy stuff our FAL barrels have working on them and it's amazing they shoot as well as they do. Lends some credence to the muzzle thingy testing and theories we played with though.

Varmint Al has been a fortunate man. With very few exceptions, it is extremely difficult to get a job like he had. I think California was not such a neutered place then as it is now. I could not even contemplate living there. But he has done a lot of fascinating work that would pass for state of the art even today.

Mark, I suspect that the barrel threads are single-point cut, and that you are seeing some effects of tool wear. Just my guess from this end of the internet...

Oh, and my vote is that IMBEL uses/used a large Brazilian monkey with a 6 foot cheater bar to tighten the barrels on. The ones I've done at least, the barrels always undertimed and fugging tiiiight on the stub.

For a given receiver an STG barrel or whatever would screw right in 'on time' and the IMBEL was undertime. I just turned the barrel shoulder on a lathe and got on with life. The up-side was that this would usually true the shoulder a bit.

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First, Kotengu, your formula is correct about calculating the amount of torque applied using an extension, EXCEPT, the 12 inches is not a fixed data point. It is the length of the torque wrench plus the extention length:

M1 = M2 x L1 / L2

Where:

M1 is the torque setting of the wrench.

M2 is the actual torque applied to the nut

L1 is the normal length of the wrench

L2 is the extended length of the wrench

Secondly, one must consider that the original barrels were installed to include some amount of shoulder deformation or crush. This crush is most likely what set the required torque spec. The inch models used breeching washers to achieve this spec instead of shoulder crush. Since we are not using virgin barrels, the specs we torque to may not equal or even achieve the original specs unless using a new breeching washer on an inch build.

Third, thread deformation is more about the class fit, thread type, and material type than about the loading as gunplumber posted. Yes loading does cause the deformation of the threads if the fit is not correct between the male and female side. I base this on my history of working on nuclear reactors. I never once saw deformed threads that were caused just by the torque loading. And some of those bolts were torqued in excess of 100k ft/lb. And I must make it clear that I consider seating of the threads as normal and deformation as abnormal (ie, not the same thing.).

And fourth, as Kotengu so mathmatically showed, a bullet only exerts 11 ft/lbs of torque on the barrel. But the same cannot be said about the muzzle device. First, the bullet does not contact the MD so torque is only transmitted through the surrounding gas layer. Secondly, one must calculate the impact force of the propellant gas cloud as is expands into the MD. And third, has the barrel rifling imparted a rotation to the exiting gas cloud so the impact force is at a vector to the bore?

I am not an engineer but I did sleep at a Holiday Inn Express once.

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Sorry Tac - a torque wrench is calibrated to its handle length. No matter if it has a 12" handle or a 36" handle, it still only imparts 100 ft-lbs at the ratchet when set on 100 ft-lbs. The longer handle will just make it easier to impart the same amount of torque. The 12s in the equation are just to convert the units.

You're right about the shoulder crush and thread deformation, but I think (hope!) your nuclear plant threads are built to a better standard than 50 year old mass produced rifle threads.

I think (THINK!) the forces on the flash hider from the gases would be minimal as they are free to expand in the atmosphere once they reach that point. The barrel torsional reaction is happening as shown in those great animations, and the flash hider is a dead weight. Inertia keeps the flash hider in place until the barrel moves it, but for that split second while the barrel is moving and the flash hider is not - the barrel is trying to unscrew (or screw) itself from the flash hider.

I don't know how G1 flash hiders would loosen unless that spring became worn or damaged, but you're right on the other. Keep in mind a "retention device" can also just be proper torque. If you torque a flash hider to 20 ft-lbs, there's no way the 11 ft-lbs of the bullet is going to unscrew it. Make your threads in the opposite direction as the rifling, and there is doubly no way.

Any more than 20 ft-lbs and you're just damaging your barrel.

One thing engineers are is conservative. If proper torque will hold it, they'll spec proper torque AND a key (like on the L1A1), or proper torque AND a left-hand thread.

Okie dokie...........whatcha think? I think this is pretty anecdotal igppo : ) i did a few more shots on the first target with 40ftlbs torque just to see what more heating of the barrel would do and it didnt help haha.

Two targets only difference was barrel torque. Ammo was 80's Port, guess if i pick up a poor shooter ill be looking at this first.